I'm not 100% certain that this is in fact true (not deducting 2018-19 prepay from 2017 return). The law specifically spells out the denial of income tax and yet leaves open a lot of interpretation on property tax. IRS guidance pertains to when a locality assessed the taxes. That's still kinda vague. In areas in MD where your 3 yr reassessment has already been made and per the Homestead act, you know what your taxes are limited to for subsequent years, the argument could be reasonably made that the assessment has in fact already been made for your next 1-3 years. Iow, you already know what your taxes will be capped at and henceforth what your assessment will be. Its so fuzzy and so many people have already pre paid it across the country, that you cant go wrong by paying it early. Youre gonna have to pay it anyway within the next year, and this govt is so wholly clusterf@#$%% with everything it lays its hands upon, that Id just pay it and claim it anyways and say to hell with it.